Super-Sized Moons Common?

By Bob Novella, on June 17th, 2011

Researchers are redefining the rarity of the earth-moon system. They are saying that a whopping 1 in 10 rocky planets may have a moon that’s big compared to its primary, just like our moon when compared to earth.

Using new sophisticated computer simulations, it seems that the massive impacts that resulted in our relatively huge moon may be common throughout the universe.

Scientists from the University of Zurich’s Institute of Theoretical Physics in Switzerland and Ryuja Morishima at the University of Colorado in the US simulated planet formation from gas and chunks of rock called planetesimals. They then took these results and factored them into N-body simluations to see what the chances were that large satellites could form.

They were quite surprised to find out that there was about a 1 in 12 chance of forming a planet and a satellite moon with both having more than half mass of the earth and moon respectively.

I liked how a BBC news article I read included a little bit more statistical information that just this 1 in 12 or 10 figure that was in so many other articles.The article stated that if you take into account the errors in the simulation, the full range of probabilities was between 1 in 45 and 1 in 4.

This is tied of course to the once controversial and now generally accepted theory how our moon formed…

In a nutshell, the young earth (Earth Mark 1 as its been called) collided with a mars-sized object creating a debris ring in orbit around the decimated earth. Within a hundred years this ring had coalesced into one or possibly two moons. Alas, this two-moon system, if it existed, could not last long as most simulations show them merging fairly quickly.

Luckily, this ring of debris was just outside the Roche Limit. Otherwise, the yet-to-be-born moon would have rained down on earth and we never would have known anything about it. Very disappointingly, the rings of Saturn are within the Roche Limit and will eventually fall planetside one day leaving our descendants with only pictures and videos of what once was.

Interestingly, this young moon was much closer to earth and appeared 15 times larger than it does now. Imagine that sight.

Tides of course would be concomitantly garguantian with such a close moon. Imagine tides as big as mountains scouring the surface scores of miles inland twice a day. Nasty.

On the bright side, this scouring could have potentially been instrumental in brewing that primordial soup thing we’ve all heard about.

So what good is this information about the prevalence of large moons?

Well, large moons can distort the measurements made to find extra-solar planets. This new knowledge then could make finding them easier. That’s pretty nice.

More big moons could also have interesting effect on life itself. We of course have only one datapoint on this but it does seem clear that a moonless earth would be very different from the way it is today.

For example:

The moon stabilizes the tilt of earth’s axis (its obliquity). Without this, the tilt could vary greatly; unevenly heating the earth in ways inimical to life.

As mentioned above, the ancient and destructive tides caused by the close moon could have given life a leg-up

The day would only be eight to 10 hours long since tidal breaking would have been reduced greatly if only the sun produced tides on the earth.

The winds would be much stronger on earth without a moon.

Complex life could potentially then not even exist yet.

When life did arrive, it would likely have a biology very different from what we’re familiar with.

There are however some scientists that see these findings in a bit of a different light.

Eiichiro Kokubo, for example, is a widely published planet formation expert who describes these recent findings as an “interesting estimate”. He continues by noting that there are various unknown parameters…

“which greatly affect lunar formation and evolution and thus the probability of hosting a large moon”.

“I think we should take the paper as a trial calculation based on what we know about formation of terrestrial planets and moons today”

Even if moons such as earth’s turn out to be a dime-a-dozen in the universe, I think we can all agree that we are pretty lucky that we have our Bella Luna to keep us company.

2 comments to Super-Sized Moons Common?

The Roche limit has to do with whether matter is far enough away to coalesce into a moon in the first place. At or below the Roche limit it will fail to coalesce and form rings.

I think the standard model of planetary formation predicts that all rings should eventually fall out of orbit. The reason Saturn and Jupiter still have rings after all this time seems to be because of the shepherd moons, or because they keep being fed by something. Saturn and Jupiter also have moons within the Roche limit, however, which is a bit of a mystery. As does Mars, though Phobos is probably a captured asteroid that’s expected to become ringified eventually.

I do wonder about the accuracy of our planet-formation models sometimes, given that we only have eight concrete data points to base them on, and that our real planets all seem to violate the very rules we make to explain how they should have formed.